A brown build-up has occurred over a period of use on the sole plate of my iron.
What is this?
Is it the same as the brown stains on my pots and pans?

Richard Hartshorn and Juliet Gerrard, a chemist and a biochemist
at the University of Canterbury, responded.

If your iron is rather old, it will simply be a shaped lump of iron
and the brown stain would most likely be rust. However, the
vast majority of modern irons have a Teflon (or similar) coating on the
metal plate of the iron, and, indeed, different metals may be used for
that sole plate as well. Your iron could well not be made of iron at all!

The coating is the same sort of “non-stick” coating that is used on
frying pans and other cookware, and it both protects the metal against
corrosion and makes a “slippery” surface that dirt, dust and other things
find it difficult to stick to. Difficult, but not impossible, as this is what
leads to the brown stain. An initial small piece of some material may
get caught on the hot surface of the iron and then be cooked or
decomposed by the heat. More dust or dirt sticks on or near the
decomposed (and possibly sticky) material and the brown stain gets
larger.

So yes, the brown stain is similar, but not the same as what you get on
your pots and pans. Cooking involves a huge variety of chemical
reactions in which beneficial flavours and aromas are produced,
and food components are made more digestible. Unfortunately there
are sometimes also side reactions in which nutritional quality is lost
and toxic compounds may be produced. In such cases, the food is
eventually rendered unpalatable.

When a pot boils over, the temperatures to which the food involved
is subjected are far higher than required for beneficial cooking effects,
and a series of uncontrolled reactions takes place, in which the food
eventually burns. This process is too complex to have been properly
understood, although some research has been carried out on what occurs.

Put simply, a series of dehydration, condensation and oxidation reactions
occur, eventually producing brown and charred residues. These organic
residues are carbon based, and tend to be tar-like, eventually turning
to charcoal (and hence smoking if reheated later). Once again a brown
mess is produced, but much more quickly than the kind of build-up that
you have seen on your iron.

If you want to clean your iron or the non-stick pans, the most important
thing is not to scrub or scratch so hard that you damage the coating.
Any defects in the coating will lead to much faster and worse build-ups
of the brown stain. Gentle scrubbing with soft cloths, and perhaps using
warm vinegar rather than water, seems to be recommended, but others
may have better suggestions.